New results from the South China Sea Monsoon Experiment (SCSMEX) have moved researchers a step closer to being able to forecast the summer monsoon and help flood planners, water managers, and farmers reduce losses of life, livestock, agriculture and property. Researchers working on this international scientific field campaign have uncovered clues to the cause, timing and evolution of the massive East Asian summer monsoon, unusual variations of which can cause devastating floods in Southern China.
Results from the study that included Asian, American and Australian scientists, will be presented at a June 2 press conference during the 1999 American Geophysical Union spring meeting in Boston. The press conference will be held at 11 a.m. in the Hynes Convention Center. Scientific presentations will be made June 1 and 2.
"The study found a connection between the disastrous 1998 Yangtze River flood and the timing and strength of the summer monsoon," stated William Lau, a senior atmospheric scientist from NASA Goddard Space Flight Center (Greenbelt, Md.) and co-chief scientist of SCSMEX. "In 1998, the South China Sea monsoon came late and with less than its usual punch, a possible warning of the intense rains over South China and the deadly floods that followed." The 1998 flooding of the Yangtze killed more than 3700 people and damage was estimated at $30 billion.
The SCSMEX uncovered clues announcing the arrival of the summer monsoon based on sea surface wind patterns seen in satellite data over the Indian Ocean and the South China Sea. Lau said that prior to the summer monsoon onset, storm activity over the Indian Ocean increased. The storms were in the form of massive twin cyclones, a phenomenon recognized a decade ago that happens every spring with varying intensity. The 1998 SCSMEX experiments marked the first time the system could be linked to the start of the Asian monsoon season, Lau said.
The cyclones, each twice the size of Texas, travel eastward straddling the equator. A few days before the onset of the South China Sea monsoon, the twin cyclones split up, with the Southern Hemisphere cyclone dying out and the Northern Hemisphere cyclone moving northward and turning into a monsoon over the Bay of Bengal.
Using information from satellites, Lau plans to watch the twin cyclone system unfold this year to try and understand how the strength and timing of the double cyclone system can help forecast flooding in Southern China.
In the South China Sea region, there is a winter monsoon and summer monsoon. During the winter, cold, dry air flows into the region from the Arctic across Asia. "At some time during the spring season, the flow reverses and brings warm, wet air from the south," said Richard Johnson, an atmospheric scientist from Colorado State University. But the transition takes place at a different time every year, prompting scientists to find ways to look for the monsoon's first appearance.
Exact timing of the summer monsoon onset is important to know for agricultural applications, water management, and flood planning because the spring change in monsoon direction represents the start of the rainy season.
Scientists used sensors on the Taiwanese island of Dongsha and buoys deployed in the South China Sea to monitor both the ocean and atmosphere. The data helped researchers to better pinpoint the start of the summer monsoon in the northern part of the South China Sea and the southern part of the South China Sea. "This sequence is unusual in the sense that monsoon rainfall is generally expected to move from south to north following the movement of the sun," said Lau.
Combining data from a radar aboard ships and a also on an nearby island, Robert Cifelli, a Goddard atmospheric scientist, is using a dual Doppler radar method to study cloud and rain patterns prior to and during the monsoon onset. The dual Doppler radar data will be combined with rainfall measurements from NASA's Tropical Rainfall Mapping Mission (TRMM) satellite to create rain maps for the South China Sea region by July 2000. Getting good rainfall estimates from a satellite vantage point is an important factor for scientists studying the Earth's climate, because when clouds condense into rain, heat is released into the atmosphere, changing wind and weather patterns.